Impacts of Climate Change

Rising global temperatures are expected to raise sea level and change precipitation and other local climate conditions. Changing regional climate could alter forests, crop yields, and water supplies. It could also threaten human health, and harm birds, fish, and many types of ecosystems. Deserts may expand into existing rangelands, and the character of some of our National Parks may be permanently altered.

Most of the United States is expected to warm, although sulfates may limit warming in some areas. Scientists currently are unable to determine which parts of the United States will become wetter or drier, but there is likely to be an overall trend toward increased precipitation and evaporation, more intense rainstorms, and drier soils.

Unfortunately, many of the potentially most important impacts depend upon whether rainfall increases or decreases, which cannot be reliably projected for specific areas.

Click here for more information about potential impacts of climate change on infrastructure.

Warming

Global temperatures are rising. Observations collected over the last century suggest that the average land surface temperature has risen 0.45-0.6°C (0.8-1.0°F) in the last century. The surface of the ocean has also been warming at a similar rate. The rate of global warming over the last 50 years is almost twice that over the last 100 years (0.13°C versus 0.07°C).

Surface temperatures are not rising uniformly. Night-time low temperatures are rising on average about twice as rapidly as daytime highs. The winters in areas between 50 and 70° North Latitude (the latitude of Canada and Alaska) are warming relatively fast, while summer temperatures show little trend. Urban areas are warming somewhat more rapidly than rural areas, because of both the changes in land cover and the consumption of energy that take place in densely developed areas (a feature known as the "urban heat island" effect).

In the United States, temperatures in the last 50 years have cooled in the East while warming in the West. Over the last 100 years, the pattern is similar, except that New England is warmer than 100 years ago because it warmed more in the first half of the 20th century by more than it cooled in the second half. This pattern of warming and cooling may be part of a worldwide pattern: while most of the earth has warmed, the regions that are downwind from major sources of sulfur dioxide emissions have generally cooled.

Precipitation

Precipitation has increased by about 1 percent over the world's continents in the last century. High latitude areas are tending to see more significant increases in rainfall, while precipitation has actually declined in many tropical areas.

In North America, precipitation has increased significantly. Precipitation in the United States has increased by an average of about 5 percent in the last century. Along the northern tier states and in Southern Canada, rainfall has increased 10-15 percent. Much of the increase in rainfall has been taking place between September and November. Rainfall is also tending to be more concentrated in heavy downpours, according to studies by the National Oceanic and Atmospheric Administration NOAA. At the beginning of the 20th century, only 9 percent of the nation experienced a storm each year in which more than two inches of precipitation fell in a 24-hour period. In recent decades, such a severe storm has occurred each year over close to 11 percent of the nation.

Sea Levels

Sea level has risen worldwide approximately 15-20 cm (6-8 inches) in the last century. Approximately 2-5 cm (1-2 inches) of the rise has resulted from the melting of mountain glaciers. Another 2-7 cm has resulted from the expansion of ocean water that resulted from warmer ocean temperatures. The pumping of ground water and melting of the polar ice sheets may have also added water to the oceans.

Along most of the U.S. coast, sea level has been rising 2.5-3.0 mm/yr (10-12 inches per century). Nevertheless, the rate varies from about 1 cm per year (three feet per century) along the Louisiana Coast, to a drop of several millimeters per year (a few inches per decade) in parts of Alaska. The rapid rate in Louisiana resulted from the settling of newly created land formed by the sediments that washed down the Mississippi River. In Galveston, the removal of groundwater led the land above the water table to sink. In areas that were covered by glaciers during the last Ice Age, by contrast, the land is rising because of the removal of the weight of the ice, which had previously compressed the land downward. As a result, the sea is dropping relative to these coasts.

Health and Environmental Effects

Climate change affects people, plants, and animals. Scientists are working to better understand future climate change and how the effects will vary by region and over time.

Scientists have observed that some changes are already occurring. Observed effects include sea level rise, shrinking glaciers, changes in the range and distribution of plants and animals, trees blooming earlier, lengthening of growing seasons, ice on rivers and lakes freezing later and breaking up earlier, and thawing of permafrost. Another key issue being studied is how societies and the Earth's environment will adapt to or cope with climate change.

In the United States, scientists believe that most areas will to continue to warm, although some will likely warm more than others. It remains very difficult to predict which parts of the country will become wetter or drier, but scientists generally expect increased precipitation and evaporation, and drier soil in the middle parts of the country. Northern regions such as Alaska are expected to experience the most warming. In fact, Alaska has been experiencing significant changes in climate in recent years that may be at least partly related to human caused global climate change.

Human health can be affected directly and indirectly by climate change in part through extreme periods of heat and cold, storms, and climate-sensitive diseases such as malaria, and smog episodes. For more information on the environmental effects related to climate variability and change, please visit the Environmental Protection Agency's website on Health and Environmental Effects.